Gas scouring systems have been used with membrane filtration systems to clean membranes and allow such systems to operate effectively over long periods with a reduced need for regular backwashing or high backwashing efficiency. Such systems typically employ pressurized gas generated by a blower or pump to produce gas bubbles which flow along the membrane surfaces and scour accumulated solids and impurities therefrom. The need for a pressurised source of gas normally requires an expensive pump and a power supply. While this is not of concern with large commercial systems, the cost of the gas supply is seen as disadvantageous to be able to provide such scouring methods to smaller systems, for example home filter systems. Such advantages are seen as having application in remote areas (farms, remote villages, expeditions) where it is hard to get electricity and pressurized air but where sufficient water pressure (about 50 to 400 kPa) is available to operate the system.